The common objective for this group of investigations is to define certain characteristics of human milk which exert specific effects on newborn infants and to examine possible mechanisms for the production of the effects. Both harmful and beneficial effects are to be sought. Properties found in human milk also will be sought in proprietary formulas. There are six component research projects in this program project. Each project addresses a unique aspect of the role of human milk in infant nutrition, maturation, and/or well-being. The specific areas to be investigated are: (1) identification and characterization of human milk lymphocytes by membrane marker and functional potential techniques, and elucidation of the mechanisms by which cell-mediated immunity is transferred via colostrum; (2) investigation of specific cellular and humoral components of milk which protect the nursing infant from two respiratory and two gastrointestinal pathogens, and investigation of colostral cellular cytotoxicity and the inhibition of natural killer cytotoxicity by colostrum; (3) characterization of the functional and metabolic activity of milk phagocytic monocytes and polymorphonuclear leukocytes; (4) a prospective study of the incidence of diarrheal disease among infants who are being breast-fed compared to a control group; (5) analysis of the protein, carbohydrate, lipid, vitamin D, and trace metal composition of milks from various categories of lactating women; and (6) a study of the effects of the chemical and physical properties of human milk on the growth of the neonatal gut and on the maturation of certain physiologic functions in newborn dogs and human subjects. Two populations of different cultural, socioeconomic, and nutritional intake are being compared, one in Houston and a second in Mexico City.